Water clarity in Lake Mendota since 1900: responses to differing levels of nutrients and herbivory

Abstract

Water transparency, phosphorus concentrations, and herbivory in eutrophic Lake Mendota have been highly variable this century. To assess the effects of both nutrients and herbivory on water clarity, mean Secchi disk readings were computed for six seasonal periods for each year during 1900-1993 that had readings (71 years but not in all periods). We assigned each year one of three levels of nutrients and one of two levels of herbivory on the basis of data independent of Secchi readings to avoid circularity in our analyses. For each seasonal period, we tested for differences in Secchi readings attributable to nutrients, herbivory, and their interaction. Our results indicate that in years with high herbivory, water clarity was significantly greater during all periods including summer when blue-green algal blooms frequently occurred. Higher nutrient levels only led to decreased water clarity during the summer and destratification periods; water clarity was not affected by nutrient levels during other periods. Our analysis of Lake Mendota's long-term Secchi disk record indicates that herbivory can strongly influence water clarity in large eutrophic lakes, and that the greatest improvements in summer water clarity in those lakes may be achieved by combining phosphorus reduction programs with biomanipulation techniques to increase herbivory.Résumé : La limpidité de l'eau, la concentration du phosphore et la phytophagie ont considérablement varié dans le lac Mendota, un lac eutrophe, au cours de ce siècle. Afin d'évaluer les effets de la concentration des nutriants et de la phytophagie sur la limpidité de l'eau, nous avons calculé les moyennes de lectures faites au disque de Secchi en fonction de six périodes saisonnières pour toutes les années entre 1900 et 1993 où ces renseignements existent (soit pour 71 ans, mais pas pour toutes les périodes). Nous avons attribué à chaque année un niveau de nutriants sur trois possibles et un niveau de phytophagie sur deux possibles en nous appuyant sur des données indépendantes des lectures au disque de Secchi afin d'éviter la création de cercles vicieux dans nos analyses. Pour chaque période saisonnière, nous avons cherché des différences dans les lectures au disque de Secchi qui soient attribuables aux nutriants, à la phytophagie et à leurs interactions. Nos résultats révèlent que, les années d'intense phytophagie, l'eau était significativement plus limpide à toutes les périodes, y compris l'été, alors qu'il s'est souvent produit des proliférations d'algues bleues. À des concentrations supérieures de nutriants, le seul effet observé était une perte de limpidité durant l'été et pendant les périodes de déstratification. La limpidité de l'eau n'a pas été modifiée par la teneur en nutriants durant les autres périodes. Notre analyse des lectures à long terme au disque de Secchi de la limpidité de l'eau du lac Mendota indique que la phytophagie peut fortement influer sur la limpidité de l'eau des grands lacs eutrophes et que les plus grandes améliorations sur ce plan au cours de la péri...

Trends and abrupt changes in 104 years of ice cover and water temperature in a dimictic lake in response to air temperature, wind speed, and water clarity drivers

“…For both air temperature and Lathrop et al (1996). wind speed, the averages from 1 November to 31 December (N-D) give the highest correlation with ice-on date; in agreement with the results of Assel and Robertson (1995) and similar to Gao and Stefan (1999).…”

“…Lathrop et al (1996) compiled Secchi depth data for Lake Mendota between 1900 and 1993 (1701 daily Secchi depth readings from 70 calendar years), and summarized the data for 6 seasonal periods: winter (ice-on to ice-off date), spring turnover (ice-off date to 10 May), early stratification (11 May to 29 June), summer (30 June to 2 September), destratification (3 September to 12 October), and fall turnover (13 October to ice-on date). After 1993, Secchi depths were obtained from the North Temperate Lakes -Long Term Ecological Research (NTL-LTER) (https://portal.lternet.edu/nis/home.…”

“…For instance, the correlation coefficient between onset date and the mean air temperature averaged over the prior month is expected to be high simply due to the seasonal variations over the course of the year, i.e., the later in the spring, the warmer air temperature. Seasonal Secchi depths were compiled for six seasonal periods: winter, spring turnover, early stratification, summer, destratification, and fall turnover (Lathrop et al, 1996), and the period closest to the averaging period used for air temperature and wind speed was used for correlation analysis. The averaging periods used for calculating correlation coefficients of each pair of variables are listed in Table 3.…”

“…Air temperature (Findlay et al, 2001;Lynch et al, 2015), wind speed (Brown et al, 1993;Lynch et al, 2015), and water clarity (Arhonditsis et al, 2004b;Lathrop et al, 1996) are important factors driving these lake ecosystem properties. Understanding how lakes respond to changes in these drivers is of great interest to predict how lakes may change in the future (Fang and Stefan, 2009;Magnuson et al, 1997;Robertson, 1989).…”

Zooplankton seasonal dynamics in a recently filled mine pit lake: the effect of non-indigenous Daphnia establishment

“…Presence of strong herbivory by Daphnia has been observed to exert greater depletions in algal concentrations than by seasonal changes in nutrient levels elsewhere (e.g. Lathrop et al 1996).…”